The invention relates to a method for cyclic communication between communication stations provided for controlling or monitoring a technical process, via a bus, in which communication sessions planned for the communication stations are executed during in each case one bus cycle.
Such a communication method is known from the standard EN 50 170 or PROFIBUS standard. The PROFIBUS is a so-called field bus which is used for communicatively linking communication stations provided for automating a technical process. A communication station at this or a similar bus is, e.g., a so-called stored-program control (SPC). A further communication station at the bus is, e.g., a so-called decentralized peripheral device to which external sensors or actuators can be connected for controlling or monitoring the technical process.
Controlling a technical process frequently also includes closed-loop control tasks. In this context, closed-loop control comprises picking up a measurement value from the technical process and outputting control information to the technical process. Both the picking-up of the measurement value and the outputting of the control information is usually cyclic. Since the measurement value is thus available not continuously but only in each case at the time when it is picked up, i.e. at the sampling time, this is a sampled-data control system, the quality or stability of which is primarily dependent on the equidistance of the sampling times.
Frequently, the measurement value is picked up from the process by a first communication station and processed by a second communication station. Using the measurement value, this second communication station also generates the control information to be output. The control information is then output to the process by a third communication station. The distance between two sampling times, and, therefore, the sampling frequency is thus determined by the duration of the communication between the respective communication stations.
To ensure equidistance of the sampling times, a constant bus cycle time is provided, e.g. in the PROFIBUS. The bus cycle time is the time interval within which all cyclic communication sessions planned for the communication stations connected to the bus are executed exactly once. A communication method with constant bus cycle time is known, e.g., from German patent application 199 39 182 (date of application 20, Aug. 1999).
A communication comprises the transfer of a message via the bus from the transmitting communication station to the receiving communication station. The time required for transferring a message is essentially determined by the volume of data transferred. The volume of data of cyclic communications, however, is essentially constant. Thus, an approximate equidistance between the individual communications is obtained with a constant bus cycle time. The approximate equidistance of the individual communications is accompanied by an approximate equidistance of the sampling times because the measurement value picked up from the process is a data item of a communication or, respectively, a message.
To ensure actual equidistance, the bus cycle time is longer than the time which would be required for executing all planned communications. The additional time is available as reserve for message retransmissions and so-called acyclic messages. If no message retransmissions are required in a bus cycle or there are no acyclic messages ready for transfer, the system still waits until the predetermined bus cycle time (including the spare time) has elapsed before it begins the next bus cycle. This results in a fixed timing pattern for the planned communications, by which the equidistance of the samples can be ensured.
The disadvantageous factor in this known communication method is, however, that this equidistance is no longer guaranteed in the case of a disturbed communication session. According to EN 50 170, in the case of a disturbed communication session, this session is repeated between once (1 time) and fifteen times (15 times) in the same bus cycle. This leads to the duration of the bus cycle being extended by the duration resulting from the repetition of the disturbed communication session. Equidistance of individual communication sessions over a number of bus cycles—as is required, in particular, for critical sampled-data control systems—cannot be guaranteed with the known communication method in the case of communication and/or transmission disturbances.
The reason for this is that each communication session which is executed after the disturbed communication session in time in the bus cycle is offset in time in comparison with a “normal” bus cycle, i.e. a bus cycle without disturbed communication session. This also lowers the quality of a sampled-data control system. In the extreme case, even the stability of the sampled-data control system can be put in question.